Coxsackie B viruses (CBV) and Adenoviruses (Ads) are the major causes of viral myocarditis, and CBV are implicated in the pancreatitis and diabetes. All CBV and many Ads initiate infection by attachment to the coxsackievirus and adenovirus receptor (CAR), a cell surface glycoprotein whose physiologic function has not been defined. This proposal concerns both CAR's role in virus tropism and its physiologic function. We recently found that CAR is a functional component of the epithelial cell tight junction, a specialized intercellular contact that serves as a barrier to the paracellular solute movement. On polarized epithelial cells, CAR is sequestered in tight junctions; consequently, epithelial monolayers resist infection by both CBV and Ads. Some CBV also attach to a second receptor, DAF, a molecule whose role in infection has remained poorly understood. Preliminary data indicate that while DAF may alter the route by which virus enters a cell, it does not trigger conformational changes in the virion essential that are essential for infection. However, DAF is targeted to the apical surface of polarized cells, and a DAF-binding CBV variant efficiently infects epithelial monolayers; we propose that interaction with DAF provides a mechanism for virus infection at epithelial and mucosal surfaces. In the first series of proposed experiments, we will determine the functions of CAR and DAF during infection, define the route of virus entry for CAR-binding viruses and DAF-binding variants, and test the hypothesis that attachment to DAF permits CBV to cross epithelial surfaces In a second series of experiments we will define CAR's function in CBV pathogenesis in an in vivo model. In human tissues, CAR mRNA is most highly expressed in the heart and pancreas, the major CBV target organs both in humans and in animal models of infection. We will use a conditional gene targeting strategy to determine whether tissue-specific CAR expression is essential for CBV infection, and for virus-induced pathology in the heart and pancreas. In a final group of experiments we will explore CAR's physiologic functions. We have found that CAR is both a structural and functional component of the tight junction, that CAR interacts with the major junctional protein ZO-1, and that homotypic interactions between CAR extracellular domains mediate cell adhesion and contribute to junctional integrity. We will determine the structures involved in CAR-mediated homotypic adhesion, and define CAR' s interactions with other proteins important for junction formation and function.